1. Field of the Invention
The present invention relates to a reproducing apparatus for at least reproducing a signal from a recording medium and a method of adjusting a spherical aberration correction value and a focus bias value.
2. Description of the Related Art
Data recording techniques are available to record and play digital data on recording media such as optical disks including a compact disk (CD), mini-disk (MD®), and digital versatile disk (DVD). The optical disks are a general term referring to recording media constructed of a thin metal disk covered with a protective plastic film. When a laser beam is directed to the disk, a change in a reflected light beam is read.
The optical disks include play only types such as CD (compact disk), CD-ROM (compact disk read-only memory), and DVD-ROM, and recordable types such as MD, CD-R (compact disk writable), CD-RW (compact disk re-writable), DVD-R (digital versatile disk writable), DVD-RW (digital versatile disk re-writable), DVD+RW (digital versatile disk plus re-writable), and DVD-RAM (digital versatile disk random-access memory). The recordable type optical disks record data using magneto-optical recording technique, phase change recording technique, or dye-doped film change recording technique. The dye-doped film change recording technique, also referred to as write-once recording technique, causes data to be written one time only with no rewriting operation permitted. The dye-doped film change is appropriate for archival purposes. On the other hand, the magneto-optical recording and the phase change recording permit data to be rewritten and find applications to record a variety of content data including music, video, gaming, and other applications.
Recently developed high-density optical disks, called blu-ray disks, stores a vast amount of data.
The high-density optical disks including the blu-ray disk are coated with a cover film of 0.1 mm thickness in the direction of thickness, and data is reproduced (or recorded) thereon using a combination of a laser of wavelength of 405 nm (so-called blue laser) and an objective lens of 0.85 numerical aperture (NA).
As is known, a recording and reproducing apparatus for recording and reproducing data on the optical disk performs a focus servo operation controlling a focal point of a laser beam onto a disk recording surface, and a tracking servo operation controlling the laser light beam to trace a track (a bit train or a groove) on the disk.
The focus servo operation is performed to add an appropriate focus bias to a focus loop.
The high-density optical disk requires a spherical aberration correction to address a thickness tolerance of the cover film and a multi-layer recording structure. For example, an optical pickup including a spherical aberration correction mechanism employing an expander or a liquid crystal device has been developed as disclosed in Japanese Unexamined Patent Application Publications Nos. 2002-352449 and 10-269611.
Since a recording and reproducing apparatus employing an high NA lens for the blu-ray disk has a narrow margin for focus bias and spherical aberration, automatic adjustment of the focus bias and the spherical aberration is important.
A method of adjusting focus bias is disclosed in Japanese Unexamined Patent Application Publication No. 2000-285484.
A method of adjusting spherical aberration is disclosed in Japanese Unexamined Patent Application Publication No. 9-251645.
A method of adjusting both focus bias and spherical aberration is disclosed in Japanese Unexamined Patent Application Publication No. 2000-11388.
As disclosed in Japanese Unexamined Patent Application Publication Nos. 2000-285484, 9-251645, and 2000-11388, a signal is read with the focus bias and the spherical aberration correction value successively changed and a combination of a focus bias and a spherical aberration correction value providing the best evaluation value obtained from the read signal is identified in the known art. The recording and playing operation are then performed with the thus identified focus bias and spherical aberration correction value set.
In accordance with the known art, the focus bias and the spherical aberration correction value are set based on the actually measured evaluation signal. For example, if a focus error signal is offset due to aging, or if a spherical aberration takes place due to variations in the cover film thickness from disk to disk or even within the surface of the same disk, quality degradation in the recording and playing operation of the signal is controlled.
Since adjustment is performed to achieve the best evaluation value, some degree of margin covers a change in focus and spherical aberration due to temperature change or disk surface flatness distortion that could take place after the adjustment.
Adjustment of the focus bias value and the spherical aberration correction value is preferably performed with a required margin maintained as shown in FIGS. 18 and 19 just in case a change occurs in focus and spherical aberration due to temperature change or disk surface flatness distortion that could take place after the adjustment.
FIG. 18 illustrates a two-dimensional contour lines representing characteristics of a value of an evaluation signal (a jitter value herein) with the spherical aberration correction value plotted in the abscissa and the focus bias value plotted in the ordinate. FIG. 19 illustrates a three-dimensional contour lines of characteristics of the value of the evaluation signal (the jitter value) with the spherical aberration correction value plotted in the abscissa and the focus bias value plotted in the ordinate.
A required margin W is defined by a range within which the focus and the spherical aberration can shift due to the temperature change and the disk flatness non-uniformity. By adjusting the spherical aberration correction value and the focus bias value at the center position of the margin W (margin center position Pm-cent), the signal recording and playing operation quality is maintained within an assumed range in response to a change in the spherical aberration and focus subsequent to the adjustment.
In accordance with the above-described known method of adjusting the spherical aberration correction value and the focus bias value, the spherical aberration correction value and the focus bias value are adjusted to reach the best value of the evaluation signal taking into consideration the margin W.
The spherical aberration correction value and the focus bias value are thus adjusted to reach the best value of the evaluation signal. If the contour line of the valuation value is plotted in a regular ellipse extending both in a spherical aberration correction value direction and in a focus bias value direction, an adjustment point becomes equivalent to the margin center Pm-cent, and a good signal recording and playing operation quality is maintained against a change in focus and spherical aberration due to the temperature change and the disk flatness non-uniformity. More specifically, if the contour line becomes a regular ellipse, the best value of the evaluation signal, namely, the center of each contour line becomes the optimum point taking into consideration the margin.
If the contour line of the evaluation characteristics is distorted as shown in FIG. 18, the known technique of adjusting to the center of the contour line can fail to provide a sufficient margin in the focus and spherical aberration after the adjustment.
More specifically, the adjustment point with the adjustment performed to the center (peak) of the contour line becomes Pm1. With the adjustment performed to the adjustment point Pm1, characteristics of the evaluation signal are sharply degraded if a change occurs after the adjustment in the Y direction in which spacing between contour lines is narrow. As a result, a sufficient recording and playing operation quality cannot be maintained against the temperature change and the disk flatness non-uniformity.
In another technique of adjusting the spherical aberration correction value and the focus bias value in the margin, each of the spherical aberration correction value and the focus bias value is adjusted toward the margin center one dimensional direction by one dimensional direction.
As shown in FIG. 18, the spherical aberration correction value and the focus bias value are adjusted to the respective values thereof (adjustment point Pm2) in the directions thereof taking into consideration the margin responsive to the temperature change and the disk flatness non-uniformity.
Even with this technique, the adjustment cannot be performed to the margin center Pm-cent of the margin W if the contour lines of the evaluation characteristics are deformed as shown in FIG. 18. If a change takes place in the Y direction, the characteristics become sharply degraded. As a result, no sufficient signal recording and playing operation quality cannot be achieved.
The contour lines of the evaluation characteristics are particularly deformed when an optical pickup having an aberration is used or when an error value or a deviation value from an ideal value is used as an evaluation value with a partial response maximum likelihood (PRML) decoding used to binarize a playback signal. In such a case, the problem is worsened.